The
CQI mapping is clearly described in the 3GPP specifications. The retransmission is best described by
HARQ.
There are two types of physical channel retransmissions: Chase/soft combining and Incremental
Redundancy (IR). In the Chase combining case, identical bits are retransmitted. The UE stores each
transmission in soft form and adds the presently received packets to the previously transmitted packet.
This is done in order to improve decoding probability. In the IR case, varying parity and systematic
bits are used in each retransmission in order to improve overall throughput performance. An example
is given in Fig. 7.117 for illustrative purposes.
The HS-DSCH encoding procedure is given in Fig. 7.118. The transport block is appended with
CRC bits and scrambled appropriately [43]. Turbo encoding with a rate 1/3 is used. The HARQ
contains a virtual buffer used to vary the transmit information to the UE. This process will determine
what additional systematic and parity bits get transmitted to the UE. Data is segmented into
physical channels and later modulated by either QPSK or 16-QAM. Later in this chapter, we will
provide throughput performance results comparing the various modulation schemes under different
propagation conditions for multiple NodeB transmit power allocation scenarios.
Pages:
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712